1. Field of the Invention
The present invention concerns altimetry from the air or from space, especially ocean altimetry.
The invention also concerns a device for implementing the method.
2. Description of the Prior Art
As recognized during the Consultative Meeting on Imaging Altimeter Requirements and Techniques held in June 1990 at the Mullard Space Science Laboratory, the ability to carry out high precision ocean altimetry over a swath with a high spatial resolution would revolutionize many fields of earth science.
There are many methods for carrying out ocean altimetry. The three main parameters associated with these methods and defining their respective performance are: vertical precision, spatial resolution and swath.
Most conventional methods are based on the use of radar emitting signals of suitable wavelength, for example an airborne radar. There are various types of radar: single-pulse system, interferometric system, etc. Altimetry of this type has been mostly limited to nadir-looking type instruments, restricting the range of possibilities.
Some prior art methods used existing radio frequency signals and combine in an airborne or space receiver signals from the transmitter (direct signals) and signals reflected from the ocean (or, more generally, from the terrestrial surface). In the following description the expression "terrestrial surface" refers to the terrestrial crust, or to the surface of oceans, seas or lakes, or to the surface of frozen water.
The following two documents described such methods:
The article by Philipp HARTL and Hans Martin BRAUN: "A Bistatic Parasitical Radar (BIPAR)" published in "INTERNATIONAL ARCHIVES OF PHOTOGRAMMETRY AND REMOTE SENSING", vol 27, 1988, pages 45-53; and
the technical report "AIRCRAFT ALTITUDE DETERMINATION USING MULTIPATH INFORMATION IN AN ANGLE-MEASURING NAVIGATION SATELLITE SYSTEM" by David KURJAN, Moore School Report no. 72-12 of 30 Sep. 1971.
Usable existing sources of radio frequency signals, known as "sources of opportunity", include communication and television satellites such as the "GPS" ("Global Positioning Satellite") satellite navigation system used in the West and its equivalent GLONASS in the former Soviet Union.
Whilst retaining the concept briefly mentioned above (use of sources of opportunity and combination of direct and reflected waves), this invention is directed to providing a very high precision altimetry method with a specific application to estimating the height of seas of oceans and variations therein. This invention is also directed to maintaining this precision over large area swaths, the measurements no longer being restricted to the nadir.
To this end, one major feature of the method of the invention is the use of multiple correlation of the direct and reflected signals.
For simplicity, but without any intention of limiting the invention, the remainder of this description concerns signals from "GPS" system satellites and a receiver on a satellite in low earth orbit ("LEO"). The receiver can be an airborne receiver without departing from the scope of the invention.
Also for reasons of simplicity, unless otherwise indicated the remainder of this description concerns only specular reflection. Signals obtained by diffuse reflection can nevertheless be used, especially in sea ice mapping applications.
To be more precise, altimetry is performed by measuring accurately the coordinates of the point of specular reflection by delay measurements and using a geodetic Earth model.
One model that can be used is the WGS-84 ("World Geodetic System 1984) model. A definition of the WGS-84 model can be found in ANON, "Department of Defence World Geodetic 1984--its definition and relationships with local geodetic systems", Defence Mapping Agency Technical Report, No. DMA TR 8350.2, second edition, 1988.
A general description of geodetic systems is given in the article by P.A. CROSS: "Position: Just What Does it Mean!", published in "Proceedings of NAV'89 Conference, Royal Institute of Navigation, London.
Finally, if the transmitter is one of the "GPS" satellites or a source of signals of this type, its instantaneous coordinates and the characteristics of the signals transmitted are accurately known.
Until now it has been implicitly assumed that a single source of signals is used, of example the transmitter of one of the "GPS" system satellites. A plurality of spatially separate sources can advantageously be used. In this case altimetry can be carried out over several subtracks which will be spread over a distance depending on the receiver altitude.
If several receivers are assumed, as in the case of a constellation of small receiving satellites, then the subtrack distance can be determined by adjusting the orbits of the receivers.
The invention also authorizes real time processing of the data using on-board signal processing devices, necessary for implementation of the method, unlike prior art systems (using "SAR" type radars defined below, for example) which use installations on the ground to process the acquired data. This leads to significant delays in availability.